1. Field of the Invention
This invention relates to pressure sensors and, more specifically, to a packaging and high pressure sealing system for such pressure sensors.
2. Brief Description of the Prior Art
Pressure sensors containing pressure transducers are well known, typical such systems being described in U.S. Letters Pat. Nos. 4,726,492 and 4,774,626.
Pressure transducers of the prior art have generally been fabricated by providing a pressure sensing module including an electronic circuit having a variable capacitor responsive to fluid pressure. The electronic circuit sensed the capacitance of the capacitor and provided an output through a plastic electrical connector indicative of the sensed pressure. The transducer elements were arranged such that a metal cup assembly having an opening or fluid pressure inlet at one end thereof to receive the fluid under pressure to be measured also included therein the pressure sensing module. The pressure sensing module was spaced from the inlet by a gasket or O-ring, the electronic circuit and connector also being contained within the metal cup. The parts were held together within the metal cup by crimping the metal cup over the plastic connector, the latter being used as a support member.
The above described pressure transducer provided highly satisfactory results when operated in the low to moderate pressure ranges of up to 500 to 600 psi. However, when subjected to pressures at the pressure inlet in the range of about 3000 to 5000 psi and up, the above described pressure transducers tended to fail. There were several points of failure, these being mainly from the inability of the plastic connector to support the high pressures to which it was being subjected. More specifically, the crimp at the metal cup was unable to retain the electrical connector crimped thereunder with the result that the connector was moved out of the cup and caused a failure of the transducer. A further point of failure was the plastic connector itself which tends to shear or compress and release the gasket or O-ring under the high pressure and release the gas under pressure from the interior of the pressure transducer. It is therefore clear that the prior art pressure transducer of the type described hereinabove was incapable of reliable operation in a high pressure environment.
The above described problem of the prior art was materially improved in accordance with the packaging techniques described in the above mentioned Ser. No. 07/282,185, filed Dec. 8, 1988 wherein the pressure sensing element is isolated from the connector by being disposed in an all metal surrounded cavity whereby the plastic connector is separated from the pressure transducer element and is not bearing the loads produced by the high pressure fluid. This is accomplished by a metal member disposed between the pressure sensing element and the plastic connector whereby the high pressures encountered by the pressure transducer are never applied to the plastic connector.
A problem encountered in the embodiments of the above noted application having only a gasket or O-ring is that, under the high pressures encountered, the gasket or O-ring has a tendency to partially extrude between the can and the pressure sensing module due to the tolerance on the diameter of the can and on the pressure sensing element. This causes seal failure and leakage.
In accordance with a second embodiment of the above noted application, an O-ring of resilient material is provided in conjunction with a back-up ring of relatively rigid material in place of the gasket. In this embodiment, the back-up ring has a tendency to partially extrude between the can and sensing element, however it stops its extrusion rapidly and the O-ring now has a closed in area to seal against with no gaps into which it can extrude. This embodiment is further in the form of a face seal wherein the two mating parts, namely a face of the housing and the opposing fluid pressure receiving face of the pressure sensing module create a gland of a controlled volume to prevent formation or existence of any extrusion gaps. In this embodiment, the pressure sensing surface of the pressure transducer package contacts an opposing surface of the housing. Therefore, the pressure sensing element is susceptible to breakage or performance deviations due to loading thereon caused by the force applied thereto by the housing surface adjacent thereto.
The compression or loading of the back-up ring and the O-ring can vary due to part tolerances. In addition, the output of the pressure transducer varies with the loading which the O-ring applies to the sensing element. The loading due to the back-up ring also affects the pressure transducer output. It is therefore necessary to minimize this problem in order to provide continually accurate and repeatable readings.